An original sheet of separator for use in a battery, such as a lithium ion battery, can be slit along a lengthwise direction of the separator original sheet into a plurality of long separator sheets. The long separator sheets each have a certain slit width that is orthogonal to the lengthwise direction.
Each of the long separator sheets is wound around a core to form a separator roll, and the separator roll is supplied to a battery production process. In the battery production process, the long separator sheet is cut to have a certain length in a direction orthogonal to the slit width, and is used as a separator.
However, it has been known that a long separator sheet like above becomes curled at an edge resulted from slitting or at an edge parallel to the slit width, in a manner such that the edge lifts up or that the edge lifts up and becomes curled into a roll. A long separator sheet in which the amount of such curl is greater than a certain value is difficult to use as a separator in the battery production process.
Therefore, knowing the amount of curl in a long separator sheet of a separator roll is very important in order to judge the quality of the separator, and many attempts have conventionally been done to measure the amount of curl.
For example, Patent Literatures 1 and 2 disclose measuring the amount of curl by: cutting, from a long separator sheet, a sample having a size of 300 mm in machine direction (MD) (which is the lengthwise direction of the long separator sheet) and 200 mm in transverse direction (TD) (which is a direction orthogonal to the lengthwise direction of the long separator sheet); and measuring the maximum distance from a flat surface on which the sample is placed to a lifted edge of the sample with the use of a graduated ruler.
(a) of FIG. 13 is an illustration of a conventional method, which is disclosed by Patent Literature 1, to measure the amount of curl at an edge that is parallel to the TD. (b) of FIG. 13 is a table showing judgements of whether a battery separator is acceptable or unacceptable according to the amount of curl thus measured.
As illustrated in (a) of FIG. 13, in the conventional method of measuring the amount of curl disclosed by Patent Literature 1, the amount of curl in each of samples A to D is determined by measuring the maximum distance from a flat surface on which the sample A, B, C, or D is placed to a lifted edge with the use of a graduated ruler. In this method, in a case where the sample is apparently curled in the MD into a roll like samples C and D, the amount of curl is regarded as large and unmeasurable. In regard to samples C and D, sample D is apparently curled to a greater extent than sample C. Therefore, the amount of curl increases in the order of sample A<sample B<sample C<sample D.
As shown in (b) of FIG. 13, whether the separator is acceptable or unacceptable as a separator for a laminated-type battery or a wound-type battery is determined according to the above amount of curl at the edge parallel to the TD. The laminated-type battery is a battery in which a positive electrode, a separator, and a negative electrode are stacked together. The wound-type battery is a battery in which a stack of a positive electrode, a separator, and a negative electrode is rolled in a roll.
In a laminated-type battery, the separator is placed on the positive electrode or the negative electrode such that the separator covers the positive electrode or the negative electrode. Using a separator curled to the extent of sample A in a laminated-type battery is not particularly an issue; however, in a case where the separator is curled to the extent of sample B, C, or D, the separator hardly functions because some portion between the positive electrode and the negative electrode has doubly stacked separator layers and another portion has no separator therein.
On the other hand, in a wound-type battery, the separator, which lies between the positive electrode and the negative electrode, is rolled in the MD with tension applied thereto together with the positive electrode and the negative electrode. Therefore, using a separator curled to the extent of sample A, B, or C is not particularly an issue. However, in a case where the separator is curled to the extent of sample D, the separator is difficult to use because this separator is difficult to roll.
As described earlier, the amount of curl at an edge parallel to the TD is important in order to judge the quality of a separator; however, in a wound-type battery, the separator is rolled in the MD with tension applied thereto, and therefore the amount of curl at an edge parallel to the MD is of more importance than the amount of curl at an edge parallel to the TD. In view of this, it is more important to know the amount of curl at an edge parallel to the MD in order to judge the quality of the separator.
Patent Literature 2 discloses measuring the amount of curl at an edge parallel to the MD, as well as the amount of curl at an edge parallel to the TD, by cutting, from a long separator sheet, a sample having a size of 300 mm in the MD and 200 mm in the TD, by a method similar to the method disclosed by Patent Literature 1.